In-situ gas analyzer

ABSTRACT

A gas analyzer which utilizes an infrared beam which passes through or has emanated from a gas stream. The analyzer has a plurality of gas cells which contain gas of the type whose concentration is being measured in various concentrations and at various total pressures. A beam of energy is forwarded to the analyzer, and is passed selectively to the various cells. In one embodiment, the cells are mounted where a rotatable deflector can selectively deflect and receive the beam to and from two fixed beam segments so that sequential readings at an infrared detector can provide measurements of concentrations, and calibration. In another embodiment, the cells are selectively moved into the beam path. The beam is formed and forwarded by a Cassegrainean system or by a combination of reflectors including a retro-reflector. An infrared calibration beam which does not pass through the gas stream is utilized for calibration.

CROSS REFERENCE TO OTHER PARENT APPLICATION

This is a continuation-in-part of applicant's copending U.S. patentapplication, Ser. No. 470,146, filed Feb. 28, 1983, entitled "In-SituGas Analyzer", now abandoned.

FIELD OF THE INVENTION

This invention relates to instrumentation to measure concentration of acomponent gas in a gas stream, for example, the concentration of carbonmonoxide in the exhaust gas stream from a gas-fired, coal-fired, oroil-fired boiler.

BACKGROUND OF THE INVENTION

Operators of combustion devices such as boilers have become increasinglyaware that continuous detection and measurement of gases produced inminor quantities such as carbon monoxide, and responsive control of theprocesses which produce them, can result in dramatically improved fuelefficiency. For example, the provision of excess air was formerly widelyused in combustion processes on the assumption that a lean mixture wouldassure more complete combustion of the fuel. However, as combustionprocesses became better understood, it also became apparent that the useof excess air was wasteful, because among other things it required theflame to heat excess gas, enabled the formation of SO₃ instead of merelySO₂, encouraged the formation of NO, created sulfate emmissions, and insome cases even increased smoke formation by shortening the flamelength. Combustion operations using low excess air improve all of theabove situations, but the control must be accurate, and be quicklyresponsive in order to insure complete combustion while avoidinguneconomical operations and the formation of excessive pollutants. Theconcentration of carbon monoxide produced by a combustion process turnsout to be a good measure of the average combustion quality, i.e.,nearness to stoichiometric condition. No CO means too much air, whilehigh CO means not enough air.

With the realization that controls based on the concentration of someminor component of a gas stream can lead to an optimized combustionfunction, serious development of suitable instrumentation wasundertaken, especially instrumentation for measuring the concentrationof carbon monoxide in a gas stream. Of course, measuring techniques andinstruments had long existed for this purpose, but frequently theyrelied on sampling techniques which were too slow to provide useful datafor on-line adjustment of combustion parameters, or not reliable enoughfor continuous duty.

The increased stringency of government regulations relating to powerplant emissions has long been a prod for the development of in-situ gasanalyzers, and several types of such analyzers have been installed inhundreds of power plants in recent years. Some utilize the techniqueknown as "gas filter correlation", which is a technique utilized in theinstant invention. It is an object of this invention to employ thistechnique to better advantage in a gas analyzer whose sampling is done"in-situ", meaning without removal of a sample from the stream, butinstead securing data as the consequence of measurements or observationsof spectral energy which has been subjected to interaction with the gasstream itself--either by having passed through the gas stream or byhaving emanated from it.

Gas filter correlation is a well-known procedure which does not requiredescription here for an understanding of the invention. A usefulreference on this subject is "Analytical Methods Applied to AirPollution Measurements" by Stevens and Herget, Chapter 10, pages193-231, published by Ann Arbor Science, 1974, which is incorporated byreference herein for its showing of the applicable theory.

It is an object of this invention to provide a gas analyzer which canhave a direct zero and span measurement, even with process sample gascontinuing to flow through the process; which can readily andautomatically be calibrated, and all interferences automaticallyrejected; which can be constructed so as readily to be accessed forroutine repair and maintenance, and even disposed at a considerabledistance from the stack; which is sufficiently heat resistant that itsreadings do not stray during temperature excursions; which rejectsspurious signals from its surroundings; and which is forgiving ofsubstantial physical shifts and changes in the physical environment,such as by expansion and contraction.

Still further objects are to provide better techniques for internalcalibration of the instrument, for more efficient optical path, and fordecreased sensitivity to external physical distortions such as vibratoryand temperature induced dimensional shifts, as well as to interferencesfrom changing gas stream chemical composition.

BRIEF DESCRIPTION OF THE INVENTION

A gas analyzer according to this invention utilizes spectral energywhich has been subjected to interaction with a gas stream, either byhaving passed through the stream, or by having emanated from it.

The heart of this invention is an analyzer with an array of gas cellsfor reference and optionally for calibration, to which a beam ofspectral energy is directed. The beam will, before or after interactionwith these cells, also interact with the gas stream, either by beingpassed through the gas stream, or by having emanated from it. A detectoris responsive to the energy which has interacted both with the gasstream and with the reference cells (optionally also with thecalibration cells).

In one embodiment, the analyzer supports the cells relative to a movabledeflector device which is movably related to two fixed beam segments.When this small device moves, it directs one of the beams to a selectedone of the cells, receives the reflected beam from the cell and directsit along the other fixed beam segment. Thus, the analyzer operateswithin itself to direct the energy to be analyzed to selected cells, butcan be placed anywhere that it receives an incoming beam segment, whichcan be fixed, or where it can produce a beam to be passed to the stack,which beam can also be fixed. Optional means can be provided to presentdifferent cells to the beam from time to time.

Optical devices can be placed in the path of the beam at appropriatelocations to exert a focusing action which assures that regardless ofphysical shifts or movements of reasonable magnitude, the beam willfully fall onto the face of the detector. In other portions of thesystem, cassegrainean optics and/or cube corner reflectors can beprovided which also reduce sensitivity to dimensional variations.

In one embodiment, a spectral source provides a beam which is passedtwice through the gas stream (being reflected after the first pass).Alternatively, the source for one of the fixed beams may be emissionsfrom the process or from the gases themselves.

In another embodiment, the spectral beam may be passed a single timethrough the stack gases, and then received and treated by the analyzer.This embodiment may also be adapted to receive and treat a beam ofenergy derived directly from the gas stream itself, by emission, or by"observing" the process itself, such as by receiving energy from aprocess flame in a burner, or from the gaseous region above a process,such as just above the molten glass surface in a glass furnace.

The reference cells can contain mixed gases to measure parameters ofmore than one gas, whose pertinent spectra do not interfere with oneanother. Carbon monoxide and sulfur dioxide constitute one such mixture.

According to yet another preferred but optional feature of theinvention, a chopper is placed in the energy path between the source andthe sample, whereby to provide pulses of energy to the detector at afrequency determined by the chopper, thereby providing means to rejectspurious data.

According to yet another preferred but optional feature of thisinvention, a separate calibration beam path is provided which by-passesthe stream on its way to the analyzer in order to give a zero-basedreading.

According to still another preferred but optional feature of theinvention, a pair of cube-corner retro-reflectors are provided to returnthe beam, one on each side of the gas stream, one to return the beamacross the stream, and the other to return it in a calibration modewithout crossing the gas stream.

According to yet another preferred but optional feature of theinvention, the cells used for calibration have two separate gas chamberscontaining gases at different concentrations and pressures in order toprovide two sets of data for the solution of two simultaneous equations.

The above and other features of this invention will be fully understoodfrom the following detailed description and the accompanying drawings,in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of the invention;

FIG. 2 is a top view of FIG. 1;

FIG. 3 is a fragmentary view of FIG. 1, operating in a calibration mode;

FIG. 4 is a top view of another embodiment of the invention;

FIG. 5 is a fragmentary view of yet another embodiment of the invention;

FIG. 6 is a fragmentary view of still another embodiment of theinvention;

FIG. 7 is a fragmentary schematic view showing means for placing theinstrumentation at a greater distance from the gas stream;

FIG. 8 is a partially-schematic axial view of the presently preferredembodiment of the invention;

FIG. 9 shows an alternate means to present various cells;

FIG. 10 is a cross-section taken at line 10--10 in FIG. 9;

FIG. 11 is a fragmentary enlargement of part of FIG. 8; and

FIG. 12 shows an alternate sampling technique.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 and 2 show an illustrative and useful embodiment of thisinvention, installed so as to measure the concentration of a selectedgas in a gas stream. This example is a double-pass instrument, the beampassing twice through the gas being sampled. In the example given, thegas being observed is carbon monoxide. However, any other gas orsubstance subject spectrographic analysis could instead be detected andmeasured by appropriate modification of the instrument. Therefore, thescope of this invention is not to be limited to carbon monoxideanalysis.

A gas stream containing carbon monoxide (or other gas whoseconcentration is to be measured) passes through a conduit such as a ductor a stack 10 (shown schematically) from a combustion device such as aboiler (not shown), on its way to atmosphere. Such a conduit will have acontinuous peripheral wall 11 through which gas stream 12 flows.

This invention is not limited to use with gas streams in stacks orducts. Such an example is given to show the best mode contemplated forits use at the present time. It is also applicable to observation ofgases at the situs of the process, such as by analyzing spectra from theprocess itself. An example is a flame, directly observed. Anotherexample is the observation of the gaseous region above a process, forexample, above the glass surface in a glass furnace.

Two ports 13, 14 are formed through the wall of the duct, and respectivewindows 15, 16 are placed in them to provide observation access for theinstrument. The window glass should not be absorptive of wavelengths ofinterest. For carbon monoxide analysis, sapphire glass is suitable.

The windows are accessible so they can be cleaned. Because they arefrequently located at inconvenient locations, means can be provided toincrease the length of time between cleanings. One such means is anozzle manifold placed adjacent to, and just upstream from, the window.Air blown out of these nozzles forms a region of increased pressurealong the surface of the window, thereby isolating the window frommaterials in the stream which might adhere to the window and reduce thetransmission. Ultimately the windows will become excessively soiled andwill have to be cleaned, but much less frequently than if this featureis not provided.

A transceiver module 20 is mounted to the conduit wall adjacent to oneof the windows. A reflector module 21 is mounted to the stack walladjacent to the other window. In fact, if preferred, the windows can beformed as part of the modules, and can be reached for cleaning bybacking the modules away from the stack wall.

An emitter 25, of spectral energy, in this case a source of infraredenergy in the band range between about 0.5u and 10u, is mounted tohousing 26. The presently preferred emitter is a cartridge heater, butone alternate source might be a conventional home appliance igniter.Such igniters are inexpensive and durable. Despite the fact that theywere designed for intermittent usage, they perform very satisfactorilyover a long term of continuous use, glowing a dull red color, andemitting infrared energy in the said band which is useful for infraredspectroscopy.

To provide a pulsed beam, a rotating chopper wheel 30 is rotatablymounted in a path of the energy from the source. The wheel is driven bya motor (not shown) at a rotational velocity which will produce pulsesof the correct frequency. The wheel has an opaque structure 31, withtransmissive portions 32 through the structure. These portions mayconveniently be open slots, open at the edge of the structure. Theirnumber and width is selected so that, with a selected rotationalvelocity, energy pulses of the correct duration and frequency passthrough the wheel.

A divergent beam 35 of infrared energy leaves the chopper wheel, andimpinges on a two component mirror 36. In its most convenientconfiguration, the mirror is generally circular, although it need not bethat shape. If it is, it can be made quite compact, with a first centralcomponent 37 and a ring-like second component 38 surrounding the firstcomponent.

Component 38 is a collimating reflector. It collects energy whichimpinges on its front surface 39 and directs the energy in a collimatedbeam segment 40. Component 37 has a reflecting front surface 41, and itscentral axis is tipped to the extent that energy which impinges on itthat has passed the chopper wheel but was not included in the collimatedbeam impinges on baffles or other means which exclude it from thedetection or analysis parts of the system.

Beam segment 40 (which is "tubularly" sectioned) passes through the twowindows and the gas stream on the first pass through the sample gas. Itimpinges on a reflector mirror 42 which has an outer annular portionwith a reflective first surface that acts in a Cassegraine manner. Thisis to say that it first reflects the beam to a focal mirror 44. Thefocal mirror then reflects the beam back to a center portion ofreflector mirror 42, which then reflects the energy toward thetransceiver module as a collimated or even a convergent beam 45.

Beam 45 passes through the two windows and the gas stream, andconstitutes a second pass through the sample gas. A portion of beam 45impinges on component 37 of the two-component mirror. The axis ofcomponent 37 is so disposed and arranged as to direct the beam to aninitial mirror 50 (sometimes called an "initial reflector") of thehousing of an analyzer 49. The curvature of component 37 is such thatbeam 51 from component 37 is focused on the initial mirror. Forconvenience, the housing may include a mount 52 with a wall 53 shaped aspart of an axially-extending cylinder to support and align the initialmirror and other elements yet to be described. Initial mirror 50 isfixed to wall 53. Its reflected beam segment 56 will therefore also befixed.

A focusing lens 55, or set of lenses, is disposed in the path of beamsegment 56, to focus the energy toward a reflecting surface 57 carriedby a rotatably mounted deflector 58. A second reflecting surface 59 isalso carried by deflector 58. Surfaces 57 and 59 are planar, and form adihedral angle between them for a purpose yet to be explained. Abi-directional motor 60 rotatably drives deflector 58 in an oscillatorymovement between selected angular positions. The angle between surfaces57 and 59 is such that beam segment 81 from surface 59 is fixed. Beamsegments 56 and 81 are sometimes referred to as "first" and "second"fixed beam or fixed beams segments.

A detector 65 is mounted to wall 53, aligned with beam segment 81. Thecenter of rotation 66 of deflector 58 lies within the dihedral angle ofsurfaces 57 and 59.

A plurality of gas cells 70, 71, 72 and 73, whose function and detailedconstruction will be described later are fixed to wall 53 in the sameplane as the detector and the initial mirror. The axis of rotation ofdeflector 58 is normal to this plane.

Accordingly, the beam from the initial mirror impinges on firstreflecting surface 57, which reflects the beam as beam segment 75 to aselected one of the cells. The selection of cells is accomplished byrotating deflector 58 to an angular position such that beam segment 75impinges on the intended cell. Each cell has a gas-containing envelope76, a transparent window 77, and a mirror 78 facing toward thedeflector. They have gases in them which may be the same gas or adifferent gas, or a mixture of gases at the same pressure orconcentration or at a different pressure or concentration, depending onthe intended purpose. Some of them have more than one gas compartment,as will later be discussed. The alignment and curvature of mirrors 78 issuch that the beam is reflected as segment 81, which includes a focusinglens 82.

It will be observed that segments 56 and 81 are fixed and do not move.They are not necessarily directly aligned as shown, and often will notbe. Focusing by mirror 37 and by lenses 55 and 82 enables the analyzerto function accurately even if there is some shift or dislocation in thesystem outside of the analyzer. Such a dislocation might be caused, forexample, by uneven heating of the stack, which would cause somemisalignment in the system. The beam must simply reach the detector, andbe brought to a focused (not necessarily a sharp focus) size which issmaller than the area of the detector. Thus, the focused beam spot onthe detector might move around the surface of the detector, but willalways be within its active area. In calling beam segments 56 and 81"fixed" it is intended to say that even though there may be somemovement within the segment itself, it is not manipulated in theselection of the cells. Such manipulation is done only on the segmentsbetween the deflector and the cells. Deflector 58 with its first andsecond reflecting surfaces 57 and 59, directs the beam to a selectedcell, the selection depending on the angular position of the deflector,so that selected ones of the cells can be included in the sampling beamthat extends from the source to the detector. Thus, this analyzer isadapted to use gas filter correlation spectroscopy. In this technologyan energy beam is passed through a correlation gas cell (at a separatetime), instead of through a reference gas cell when it passes to thedetector, and at another time through a reference gas cell. Thus, one ofcells 70-73, say cell 70, is a correlation cell, and another cell, saycell 71 is a reference cell. Cells 72 and 73 are calibration cells.Their construction and function will be discussed below.

In the typical gas filter correlation instrument, a leak in thecorrelation cell results in a change in absorption at the line centers,causing a change in the instrument drift. As described below, thisinstrument uses multiple gas cells with fixed and known relationship oneto another. The electronics automatically check this relationship andcompensate, and can be instrumented to alarm if one cell has changedrelative to the others.

Across-the-stack instruments, with or without a cross-stack pipe orother rigid support, have demonstrated sensitivity to alignment changes.As the sun shines on one side of the stack, or when process parameterschange, temperature changes in the stack or duct wall cause differentialmovement of one side relative to the other. The optical components ofthe instant system (mirrors and lenses, especially the lenses in theanalyzer) can correct for these variations, when they are designed withthese variations in mind.

When the embodiment shown in FIGS. 1 and 2 is used, calibration of theinstrument is conducted with the use of an infrared beam from the sourcewhich beam is not passed through the gases to be sampled or measured.Instead the beam is sent directly to the analyzer, as best shown in FIG.3. FIG. 3 is a showing of the same system as that shown in FIG. 1, butillustrates a baffle system. Identical parts bear identical numbers.

In FIG. 3, baffle 70a is shown with a first aperture 71a that passes thebeam returned from the stack. A shutter 72a intersects beam 51 whencalibration is done. At that time shutter 72a is in the solid-lineposition shown. When measurement is to be done, the shutter is rotatedto the dotted-line position, which interesects a calibration beam 75anow to be described. The calibration beam passes through a secondaperture 73a in the baffle.

Calibration beam 75a is reflected from the source by mirror centralcomponent 37, and converges toward reflecting surface 59 on deflector58. This beam strikes the "back" surface of the deflector. Beam 75a isreflected to the individual ones of the cells as appropriate and isreflected back to surface 57 on the mount, which in turn reflects it toa concave calibration mirror 77a that focuses the beam on the detector.It will be observed that beam segments 75a, 79a, and 80a are fixed, butthat segments 81a and 82a move from cell to cell when mount 58 isrotated.

The basic instrument signal processing electronics are remotely locatedfrom the instrument box so as to increase their accessibility and toallow them to be placed in a temperature controlled enviroment. Theinstrument box itself has the minimum amount of electronics required tooperate the optical head. The detector preamplifier is mounted directlyat the back of the detector. The power supplies and the stepper motorcontrol, and other functions, can be located on a single printed circuitboard below the optical base plate. The output of the optical head canbe transmitted either by analog or digital means to a remote panel.

A microprocessor is used for signal processing at the remote panel. Thisincludes setting the optical path length across the stack, full scale ofthe instrument, linearization of the output, automatic calibration,temperature compensation of the data, (through the input of athermocouple readout in the gas stream), pressure compensation,adjustable high and low limit alarms, and diagnostics including powerfailure, blower failure, source failure, detector failure, stepper motorfailure, leak in a gas cell, dirty windows, high temperature alarm forthe detector, high temperature alarm for the instrument box, andelectronics failure, as examples.

Both the instrument box and the reflector box and the associated airpurge blowers and filters, as well as the junction box for power in andsignals out can all be enclosed in a weather-tight box for basicinstrument weather protection.

Yet another way to remove the more sensitive elements of the device to amore favorable environment is shown in FIG. 7. The instrument isresponsive to infrared beams that have passed through or emanated fromthe gas stream. While it is good practice to place the instrument nearto the substances it measures or reacts to, sometimes this isinconvenient. Electronic transmission of the raw data also involvesproblems.

This invention provides the advantage that the optical system can beinterrupted at various places, and coupled by optical forwarding meansof various types. The presently-preferred such optical forwarding meansis a fiber optic bundle. This is shown in FIG. 7, where a typicalfiber-optic bundle 100 having a sheath 101 and a large number of glassfibers 102 has one of its ends 103 fitted in the aperture in ring-likecomponent 38, in place of central component 37. Its bundle receivesenergy from beam 45. The fibers conduct this energy to end 104, and afocusing lens 105 focuses it onto initial mirror 50, wherever it isplaced. The bundle can be bent and can be of any length so the result isto enable the deflector and cells to be placed more conveniently for theuser. The glass fibers will be coated with an external reflectingcoating in accordance with known fiber-optics techniques.

Other optical forwarding means can be used instead. An example is theclassical rod-lens telescope shown in Hopkins U.S. Pat. No. 3,257,902.However, this device does not readily accomodate bends, and may be moredifficult to employ. It does have image-forming properties superior tothose of fiber glass bundles, even of coherent fiber bundles, shouldimage properties be of interest.

The embodiment of FIGS. 1 and 2 is characterized as a double passsystem. In a double pass system, the beam is twice subjected to theeffects of the gas stream, having been directed through it, and thenreflected back through it again. In this embodiment, the source isseparate from the cell mount, and the detector is held by the cellmount.

There are circumstances in a double pass system where it is preferablefor the source to be mounted to the cell mount, and for the detector tobe located elsewhere. FIG. 4 shows one such arrangement. In theembodiment of FIGS. 1 and 2, the beam is passed through the cells afterhaving been passed twice through the gas stream.

In FIG. 4, the beam is passed through the cells before it is passedthrough the gas stream. As it happens, it is also passed twice throughthe gas stream. The net result in absorption spectroscopy is the same inboth FIGS. 1 and 4. A wall 120 identical to wall 53 supports four cells,121, 122, 123, and 124, which are identical to cells 70, 71, 72, and 73,respectively. A rotatable deflector 130 having reflective surfaces 131,132 is identical to deflector 58. An infrared source 133 is mounted towall 120 where detector 65 is in FIG. 2, and a mirror 134 (forconvenience sometimes called an "initial mirror "or" initial reflector",as used for mirror 50, even though it is not the first mirror to beimpinged on by the beam) is mounted to wall 120, where mirror 50 islocated in FIG. 2. Thus, except that an infrared source has beensubstituted for the detector, this part of FIG. 4 is identical to therespective part in FIG. 2.

Fixed beam segments 135 and 136 are on opposite sides of the deflector,with respective lenses 137 and 138. Reflecting surfaces 131 and 132reflect incident energy as shown. The back walls of the cells aremirrored as before.

A concave mirror 140 reflects beam 141 from mirror 134 to a beamsplitter 142. The beam splitter is a partially reflecting mirror whichtransmits about half and reflects about half of the energy incident uponit. A chopper 143 identical in form and function to chopper 30 isinterposed in beam 141.

Beam 145 to the right of the beam splitter corresponds to the beamreflected by mirror 39 and reflected by the reflector in FIG. 2. Theportion to the right is identical to that in FIG. 2, and is not repeatedin the drawings.

A detector 146 similar in form and function to detector 65 in FIG. 2receives the treated beams.

An optional semi-reflecting mirror 147 is shown which might be such as ahot or cold mirror to deflect energy of different wavelengths to bemeasured for some purpose.

For purposes of calibration, a calibration segment 150 extends belowbeam splitter 142 to a reflecting mirror 151 that reflects the beam backto the beam splitter. The portion which is reflected to the left isreceived by the detector. The part which passes through is ignored. Acalibration chopper 152 blocks beam 145 for the calibration cycle, andthen blocks beam 150 for sample measuring.

Thus, FIG. 4 is a double pass instrument with the source and detector ineffect interchanged in position. FIGS. 5 and 6 are single passinstruments. In FIG. 5, a beam is projected through the gas stream. InFIG. 6 the energy to form the beam emanates from the gas stream itself.

FIG. 5 functions together with all of the equipment shown to the left ofthe gas stream. Instead of merely returning a beam across the gasstream, it originates the beam, utilizing an infrared source 160,chopper 161, and a reflector system such as the Cassegraine-type 162that projects a beam 163 across gas stream, where it passes throughwindow 15 and is treated by the remainder of the system in FIG. 2. Ofcourse, chopper 30 is not used at this time. Instead of the Cassegrainesystem, mirror 165 may collimate the beam directly.

When calibration is to be done, the separate system of FIG. 3 will beused, the relationship between the two sources being known.

FIG. 6 utilizes emission spectroscopy instead of absorption spectroscopyin the equivalent of a single pass system. A window 170 in the wall of aduct 171 for a gas stream 172 that emanates infrared energy passesenergy to collimating lens 173 that forms a beam 174 which impinges on afocusing mirror 175. This beam is reflected to initial mirror 50, and istreated by the remainder of the system of FIG. 2. A chopper (not shown)can be placed in the path of the beam. For calibration, the separatecalibration system shown in FIG. 3 can be provided. In all embodiments,when changing from a calibration to an active measuring mode,appropriate shutters will be moved to exclude confusing or extraneousbeams from the system. Some of these are not shown, because theirpurpose and possible locations are evident.

Cells useful in the analyzer and in the system, and their use, will nowbe described. Cell 70 is referred to as a "correlation" cell. Cell 71 isreferred to as a "reference" cell. Cells 72 and 73 are referred to asfirst and second "calibration" cells, respectively.

Cell 70 has a single gas-tight compartment 200. It contains gas of thetype being measured, for example carbon monoxide at a partial pressureof generally the same partial pressure as the substance exists in thesample being measured, and another gas as a broadening agent, forexample, nitrogen. The other gas increases the total pressure in thecompartment 200 to a pressure that is subatmospheric, and such that theline widths in the spectrum from this cell in use will be about the sameas the line widths in the spectrum from the sample in the process beingmeasured. In use, correlation cell 70 provides a measurement ofbackground intensity.

Reference cell 71 has a single gas-tight compartment 201. It contains aquantity of the gas being measured at about the same partial pressure asin cell 70. It also contains broadening agent, the same as in cell 70,but this agent is supplied in an amount such that the total pressure isgreater than in cell 70. This broadens the lines giving a similar totalabsorption, but a significantly lower absorption at the line centers.

First calibration cell 72 has two gas tight compartments 202, 203, whichare serially located in the beam, with a transparent wall 204 betweenthem. Compartment 202 contains a known partial pressure of the "sample"gas, i.e., a known partial pressure in a total pressure (in the presenceof nitrogen gas, for example) at a subatmospheric pressure. Compartment203 is equivalent to the reference cell, and contains "sample" gas atapproximately the same partial pressure and total pressure as in cell71.

Second calibration cell 73 has a first and second compartment 205, 206respectively, as in cell 72. Compartment 205 contains the same gases ascompartment 203, but with the sample gas at a known, higher partialpressure. Compartment 206 contains the same gases at the same pressuresas compartment 204.

Broadly stated, the filling of correlation cell 70 is such that theabsorption line widths in the correlation cell, which is at the ambienttemperature of the instrument box, are essentially identical to the linewidths in the gas stream containing the sample. Applications for thisinstrument will normally range from gas stream temperatures of about250° up to about 750°. Applications outside of this temperature rangeare also possible. Sufficient absorbing gas partial pressure is utilizedin the correlation cell to insure essentially complete absorption of theline centers at those wavelengths where the sample gas absorbs. With anarrow band pass filter in front of the detector which transmits energyonly in wavelength band (Δλ) where the sample absorbs, then the onlyenergy seen by the detector when the correlation cell is in the beam isthat energy which is transmitted through the gas stream at thoseinterleaving wavelengths where the sample does not absorb.

The reference cell will be filled to a higher total pressure such as 5atmospheres. This causes absorption at the same wavelengths, but becauseof the higher total pressure the absorbing lines are much broader. Thedetector then alternately sees a beam which passed through thecorrelation cell with complete absorption of sample, giving only thebackground radiation, and then the beam which has passed through thereference cell, giving the background radiation plus a partialabsorption of CO (caused by the CO in the stream and in the cell). Sincethe background is the same through both cells, the change in absorptionby the sample in the reference cell is directly proportional to theconcentration of the sample in the gas stream.

The calibration cells provide an up-scale instrument calibration point.With the zero and span information, a microprocessor based set ofelectronics (which is located remotely from the analyzer) can provide aperiodic automatic full calibration and output adjust in accordance withknown procedures.

The two calibration cells differ from the correlation and referencecells in that each one has two compartments. One compartment containsthe low concentration calibration/span gas, and the other compartmentcontains the equivalent of the reference cell, but at knownconcentrations. Therefore, although the beam does not pass through thesample, this effect is reproduced on a known concentration. Thus, thebeam passes through the reference cell and a known span concentration,and then to the detector. The signal seen at the detector is compared tothat seen after the beam has passed through the correlation cell fordirect calibration.

As part of a calibration cycle, the sample beam is sequentially steppedthrough each of the two calibration cells. This provides twoincrementally added known concentrations to the unknown stackconcentration. These two additional data points allow the computationand elimination of zero and span offsets, such as the measurement of CO.These are H₂ O and CO₂. This calibration cycle is done frequently, at aselected frequency, which may be adjustable. Five minute intervals aregenerally satisfactory.

FIG. 8 shows the presently preferred embodiment of the invention. Itenables the available radiant energy more efficiently to be used, andthe instrument to be less sensitive to external distortive forces, inaddition to other advantages.

The objective of the system is, as before, to provide to an analyzer300, a beam which has passed or will pass through a gas sample, orcells. This can be, and most frequently will be, the same as analyzer50. The analyzer of FIG. 4 is also useful. There are other viableanalyzers, as will later be shown.

An infrared source 301 emits infrared energy along path segment 302. Achopper 303, which conveniently comprises a bidirectionally rotatablenotched disc 304 is in said path. The spacing of the notches and thespeed of rotation of the disc determines the chopping frequency. 900 Hzis a useful frequency when this device is used to screen out backgroundnoise and interference.

A two element deflector mirror 310 is disposed on the central axis 311of the optical system. A baffle 312 extends axially along the system todivide it into two halves. It occludes path segment 302, and dividesinto two elements the mirror 310, and a focal mirror 313.

Mirror 310 has a first reflective element 310a which reflects rays alongsegment 302 to focal mirror 313. Element 310a will be generally concave,and will direct the rays to domed focal mirror 313. Domed mirror 313will direct the impinging rays onto Cassegrainean mirror 315 along path316 which in turn will reflect them along a collimated path 317, whichis half-tubular, axially.

Path 317 exits through window 318, crosses stack 319, passes throughwindow 320, and impinges on a trihedral retroreflector 321. This is aclassical cube-corner reflector comprised of three mutuallyperpendicular mirrors. The beam represented by path 317 is thereforerotated 180°, and displaced to the other side of the plane defined bybaffle 312. The rays are returned precisely parallel, along path 325 andimpinges on the other half of the Cassegrainean mirror, which in turnreflects them to the other part of the focal mirror, which in turnreflects them along path 326 to the other side of deflector mirror 310.The shape of element 310b is such as to direct the rays in a path 327 toanalyzer 300. Rays in path 327 can be treated precisely as in FIGS. 1-7.

As before it is necessary to have an optical path which does not includethe sample in order to provide for calibration. In this embodiment, thisobjective is readily met by providing a second retroreflector 330between the Casegrainean mirror and the stack. This retroreflector isidentical to retroreflector 321. It may be placed in the way of path317, and will return the rays on that portion of path 325 which does notinclude the sample. The second retroreflector can be mounted on a slideto be removed when not desired. This is an elegant means to provide twoeffective paths, with all of the advantages available to both.

FIGS. 9 and 10 illustrate that many of the advantages of this inventioncan be attained with a different means to mount the cells, and withoutusing a rotatable deflector. In FIG. 9, a wheel 350 is shownbi-directionally rotatable around an axis 351 which is parallel to andoffset from path 327. This wheel has a plurality of ports 328, 329, 330and 331, each adapted to hold a respective cell, like cells 70-73,respectively. A detector 335 receives energy passed by the cells, and itcorresponds precisely to detector 65 in FIG. 2. It is evident that thepositions of the source and the deflector can be reversed.

In the embodiments already described, the gases in the stack aretraversed by the beam, or are used as a source. It is equally within thescope of this invention to divert gas through a sampling chamber, and touse the gases in the sampling chamber the same as the gases in thestack. FIG. 12 shows a sampling chamber 340 with an inlet 341 and anoutlet 342. Stack gases are diverted through the chamber, which isequipped with windows 343, 344. A reflector 345 is at one side, and thetransceiver 346 (the analyzer and source) is at the other. All featuresof any of the embodiments are useful with the sampling chamber, or withthe stack.

In this specification, the term "analyzer" is used for that portion ofthe system in which the beam is distributed to the cells, or the cellsplaced in the path of a fixed beam. That part of the system in which thecalibration beam is formed, or the beam passed through the sample, issometimes called "beam forwarding means."

The operation of the system as to absorption or emission, utilizing theprinciples and practices of gas filter correlation will be recognized bypersons skilled in the art. This instrument and the applications itenables, is rugged and involves few moving parts. Importantly, it isforgiving for changes in alignments, and can be constructed if desiredso as to require only the movement of beam segments, rather than cells,as a consequence of the simple stepping movement of a mirror-bearingdeflector.

Spectral energy, i.e., wave type energy subject to absorption oremission interaction can be used, over the full range of spectralwavelengths, including ultraviolet, visible, and infrared. Of course, anappropriate emitter and responsive deflector must be provided. For mostgas measurements, the infrared region is very suitable, and detectorsresponsive to wavelengths in this region are well-developed. However,the invention is not to be limited to usage in the infrared region,because absorption and emission phenomena in other bands or regions arealso useful.

Systems according to this invention are elegantly simple and inherentlyrugged. The image-deflecting and forming elements are simple lenses andreflectors. Sharp images are not necessary, because it is only necessarythat the beam be related to selected cells from time to time, and thenarrive at the active surface of the detector. In some embodiments thisinstrument can be switched from calibration to measurement modes merelyby shifting a shutter, and operates within its measurement mode merelyby appropriate rotation of a mount which carries reflecting surfaces. Inanother embodiment the change of modes is made merely by shifting aretroreflector.

This invention is not to be limited by the embodiments shown in thedrawings and described in the description, which are given by way ofexample and not of limitation, but only in accordance with the scope ofthe appended claims.

I claim:
 1. In an instrument for measuring the concentration of a gas ina stream of other gases, utilizing a beam of spectral energy which haspassed through or emanated from at least some part of said stream, ananalyzer comprising:a plurality of gas cells spaced apart from oneanother; an initial reflector; a rotatable deflector, comprising a pairof deflecting reflectors rotatably carried by it, said deflectingreflectors forming an angle with one another, the center of rotation ofsaid deflector lying inside said angle; means for rotating saiddeflector; a spectral source or a spectral detector, said cells, initialreflector, rotatable deflector and source or detector being fixedlylocated relative to one another such that fixed beam segments of saidbeam are formed between the initial reflector and the rotatabledeflector, and between the rotatable deflector and the source ordetector, and an angularly movable beam segment of said beam is formedbetween each of the deflecting reflectors of the rotatable deflector andthe locus of said cells which movable beam segments can be moved fromcell to cell as the consequence of rotation of the rotatable deflector,a first of said movable beam segments extending from one of saiddeflecting reflectors to said cell locus, and the other of said movablebeam segments extending from the said locus to the other of saiddeflecting reflectors; and reflecting means adjacent to each said cellfor reflecting the beam from one of said movable beam segments to theother, after the beam has passed through the cell at least once. 2.Apparatus according to claim 1 in which said fixed beam segments includefocusing lenses.
 3. Apparatus according to claim 1 in which saidplurality of cells includes a correlation cell, a reference cell, and acalibration cell, said correlation and reference cells containing samplegas at approximately equal partial pressures, which, together withanother gas provides a total pressure which is a sub-atmosphericpressure in the correlation cell, and higher total pressure in thereference cell, and, in the calibration cell two compartments in series,one of which contains sample gas with a partial pressure aboutproportional to the anticipated concentration in the gas stream, and atotal gas pressure about equal to that in the correlation cell, and theother containing about the same partial pressure and total pressure asfound in the reference cell.
 4. Apparatus according to claim 3 in whicha second said calibration cell is provided which constitutes a twocompartment cell in which a different and known concentration of samplegas is contained at substantially the same total pressure as in the samefirst calibration cell.
 5. In combination:apparatus according to claim1; means for directing a beam through a gas stream, said means receivingsaid beam from said analyzer, or directing it to said initial reflector,and whichever of said source or detector is not carried by saidanalyzer, disposed on said beam spaced from said analyzer.
 6. Acombination according to claim 5 in which said detector, cells, initialreflector, reflecting means and rotatable deflector are fixedly mountedto common structure, and in which said beam is passed twice through saidgas stream.
 7. A combination according to claim 5 in which a separatecalibration beam path is provided from said source to said deflector toprovide a beam to or from said analyzer which does not pass through thegas stream.
 8. A combination according to claim 5 in which said sourcecomprises the gas stream itself.
 9. A combination according to claim 5in which chopper means is disposed in the path of said beam.
 10. Acombination according to claim 5 in which beam forwarding means isprovided in the path of said beam to enable the analyzer to be locatedat an arbitrary location relative to said gas stream.
 11. A combinationaccording to claim 5 in which said fixed beam segments include focusinglenses, which maintain the image on said detector, even when theincident angle of the beam changes.
 12. A combination according to claim5 in which said means to rotate the deflector is a stepping motor.
 13. Acombination according to claim 7 in which said plurality of cellsincludes a correlation cell, a reference cell, and a calibration cell,said correlation and reference cells containing sample gas atapproximately equal partial pressures, which, together with another gasprovides a total pressure which is sub-atmospheric pressure in thecorrelation cell, and a higher total pressure in the reference cell,and, in the calibration cell two compartments in series, one of whichcontains sample gas with a partial pressure about proportional to theanticipated concentration in the gas stream, and a total gas pressureabout equal to that in the correlation cell, and the other containingabout the same partial pressure and a total gas pressure about equal tothat in the reference cell, and a second said calibration cellcomprising a a two compartment cell in which a different and knownconcentration of sample gas is contained at substantially the same totalpressure as in the same first calibration cell.
 14. In an instrument formeasuring the concentration of a gas in a stream of other gases,utilizing spectral energy which has passed through or emanated from atleast some part of said stream:an analyzer comprising a plurality ofcells through which a beam of said energy is to be passed; beamforwarding means to direct an energy beam through or from said stream tosaid analyzer said beam being fixed as it enters said analyzer; andmeans in said analyzer to cause said beam to encounter selected ones ofsaid cells; said beam forwarding means comprising a pair of axiallyspaced apart Cassegrainean reflecting systems.
 15. In an instrument formeasuring the concentration of a gas in a stream of other gases,utilizing spectral energy which has passed through or emanated from atleast some part of said stream:an analyzer comprising a plurality ofcells through which a beam of said energy is to be passed; beamforwarding means to direct an energy beam through or from said stream tosaid analyzer, said beam being fixed as it enters said analyzer; andmeans in said analyzer to cause said beam to encounter selector ones ofsaid cells; said beam forwarding means comprising a two-element mirror,a focal mirror, and a Cassegrainean mirror so disposed and arranged thatone element of said two element mirror and one half of said focal mirrorand said Cassegrainean mirror form a substantially collimated beamoccupying substantially an entire half-tubular path, a singleretrodirective reflector in said half-tubular path which returns androtates by 180 degrees the entire energy beam which it receives, anddirects it to the other one half of the Cassegrainean mirror, whichreflects it to the other half of the focal mirror and the other elementof the two-element mirror, which in turn forwards the energy beam to theanalyzer as said fixed beam.
 16. Apparatus according to claim 15 inwhich said elements and Cassegrainean mirrors are concave, and the focalmirror is convex.
 17. In an instrument for measuring the concentrationof a gas in a stream of other gases, utilizing spectral energy which haspassed through or emanated from at least some part of said stream:ananalyzer comprising a plurality of cells through which a beam of saidenergy is to be passed; beam forwarding means to direct an energy beamthrough or form said stream to said analyzer, said beam being fixed asit enters said analyzer; and means in said analyzer to cause said beamto encounter selected ones of said cells; comprising a rotatable membercarrying a pair of angularly related mirrors, one to direct a beamsegment of said beam to a selected cell, and the other to receive a beamsegment of the beam returned from the cell, and to direct it along afixed path, the cells, reflecting means adjacent to the cells, and therotatable member being fixedly mounted to common structure.
 18. In aninstrument for measuring the concentration of a gas in a stream of othergases, utilizing spectral energy which has passed through or emanatedfrom at least some part of said stream:an analyzer comprising aplurality of cells through which a beam of said energy is to be passed;beam forwarding means to direct an energy beam through or from saidstream to said analyzer, said beam being fixed as it enters saidanalyzer; and means in said analyzer to cause said beam to encounterselected ones of said cells; said plurality of cells including acorrelation cell, a reference cell, and a calibration cell, saidcorrelation and reference cells containing sample gas at approximatelyequal partial pressures, which together with another gas provides atotal pressure in said correlation cell which is selected so that theline widths are approximately the same as the sample line widths, asincreasing temperature and decreasing pressure narrows said lines, and ahigher total pressure in the reference cell, and, in the calibrationcell, a sample gas with a selected partial pressure about proportionalto some fraction of the anticipated concentration in the gas stream, andtotal gas pressure about equal to that in the correlation cell. 19.Apparatus according to claim 18 in which said total pressures in saidcorrelation and reference cells are subatmospheric.
 20. Apparatusaccording to claim 18 in which a second of said correlation cells isprovided with about the same partial pressure and total pressure asexists in the reference cell.
 21. Apparatus according to claim 18 inwhich said beam forwarding means includes a single retroreflector toreturn and rotate an entire beam which has passed through the gasstream.
 22. Apparatus according to claim 18 in which said beamforwarding means includes a single sample retroreflector to return androtate an entire beam which has not passed through the gas stream, saidsample retroreflector being mounted for selective insertion into andremoval from the path of the beam.
 23. Apparatus according to claim 22in which another single reflector is placed in the beam so as to rotateand return an entire beam which has passed through the gas stream.